What is function/meaning of string arg to DataReferenceImpl()?

Can anyone tell me what the purpose of the string argument to
DataReferenceImpl() is? I've seen it used in several programs like this
Jython snippet:

t_h_ref = DataReferenceImpl("t_h_ref")

The point here is that string appears to be always the same as the name of
variable in the programming language. Can be an arbitrary string? What is
it used for? What happens if I leave it blank? How about if I use the same
string for several variables? (like this Jython again)